The revolution in molecular biology along with the nearly completed mapping of the human genome provides an unprecedented opportunity to transform our understanding and treatment of human diseases. Paralleling these discoveries, the imaging sciences have made remarkable advances and have reached a stage in which anatomic and functional imaging can be achieved in vivo at submillimeter resolution in both animals and humans. These developments have created a historic opportunity to non-invasively probe cellular and molecular events associated with the ever-expanding myriad of newly identified pathology-related genes and proteins in vivo in both animals and humans. The revolution in molecular biology has expanded our understanding of the genetics and biochemistry of transformed cells. These tremendous advances have been made largely through studies of cultured cells or ex vivo studies on tumor specimens. However, it is clear that extrapolations between in vitro and in vivo situations do not always hold true. Their exists a significant opportunity to bridge this great divide between in vitro and in vivo studies in research, e.g., cancer therapies, through the development of novel molecular imaging approaches.
For example, research would be aided by in vitro and in vivo imaging of apoptosis, or programmed cell death. Apoptosis is a physiologic process important in the normal development and homeostasis of multicellular organisms. The molecular components comprising the cell death machinery have been identified. Apoptosis can be physiologically activated by the activation of death receptors (Fas, TNFR, DR4, DR5 etc) or when a cell undergoes stress. Growth factor withdrawal, environmental conditions that damage mitochondrial function or homeostasis, DNA damaging events, hypoxia, heat, cold and chemical injury, result in activation of apoptosis.
Lack of balance between apoptosis and proliferation has been implicated in a wide variety of pathologic conditions including stroke, dementia, bone marrow diseases and cancer. In stroke, death of white and gray matter by apoptosis plays a central role in hypoxic-ischemic injury in adults. Because the actual loss of cells in these patients is gradual there may be a therapeutic window wherein pharmacological inhibition of apoptosis may prevent the long term debilitating effects of a stroke. In dementia, neuronal and glial cell apoptosis occurs in AIDS, encephalitis, and in neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease and ALS. The gradual loss of white and gray matter in these disorders is primarily due to apoptosis. In bone marrow diseases, beta-thalasemia, sickle cell disease and aplastic anemia are diseases in which excessive apoptosis within the bone marrow is the central cause of the pathology. The ability to image apoptosis within the marrow would not only facilitate diagnosis of these disorders but would also provide a direct measure of therapeutic efficacy of experimental drugs. Cancer is as much a disease of cell death as one of cell proliferation (see, e.g., Korsmeyer (1990) Curr. Top. Microbiol. Immunol. 166:203). For example, constitutive activation of the anti-apoptotic gene bcl-2 leads to B-cell lymphoma (see, e.g., Holgren (1995) Nature Med. 1:149). It is believed that mutations that attenuate apoptotic responses facilitate neoplastic transformation. The mutations may be allowing the accumulation of other growth-promoting mutations that would otherwise commit a cell to suicide in the absence of external growth cues. Tumor progression may also exert a selective pressure for cells resistant to apoptosis. Evolution toward a “survivor” phenotype may be a product of the hypoxia, nutrient starvation, and falling pH that may be produced as tumor cells outgrow their blood supply. The selective pressure may be especially strong just prior to the “angiogenic switch” when dormant tumor growth is thought to be the result of balanced cellular proliferation and apoptosis (see, e.g., Holgren (1995) supra). The resulting defects in the apoptotic response of tumor cells arising from early events in carcinogenesis or as a result of selective pressures are also thought to contribute to the resistance of tumor cells to cytotoxic therapies.
Currently available techniques for studying apoptosis in vivo, such as in solid tumors, make it difficult to study these problems. Scoring apoptotic indices by morphological criteria is time consuming and requires skilled observers. Specific staining of apoptotic cells, such as the TUNEL method for marking the 3′ termini of cleaved DNA, is also time consuming and may have a significant false-positive rate.